Process control systems require the accurate measurement of process variables. Typically, a primary element senses the value of a process variable and a transmitter develops an output having a value that varies as a function of the process variable. For example, a level transmitter includes a primary element for sensing level and a circuit for developing an electrical signal proportional to sensed level.
An electrical transmitter must be connected to an electrical power source to operate. One form of such transmitter, known as a four-wire transmitter, includes two terminals for connection to a power source and two terminals for carrying the output signal proportional to the process variable. Where transmitters are remotely located, the requirement for four conductors can be undesirable due to the significant cost of cabling. To avoid this problem, instrument manufacturers have developed devices known as two-wire, or loop powered, transmitters. A two-wire transmitter includes two terminals connected to a remote power source. The transmitter loop current, drawn from the power source, is proportional to the process variable. The typical instrument operates off of a 24-Volt DC source and varies the signal current in the loop between 4 and 20 milliamps DC. Because of these operating requirements, the design of the transmitter in terms of power consumption is critical. For example, when a low level signal of 4 milliamps is transmitted, there is minimal power available to be consumed by the instrument. Therefore, circuits must be designed to operate off of such minimal available power. More recently, designs have been proposed which use wireless technology for transmitting information on the process variable. Such devices may be battery powered. Again, the design of the transmitter in terms of power consumption is critical to avoid premature wearing down of the battery.
Various industrial distance or level sensing devices operate by emitting bursts of energy, usually acoustic or electromagnetic, and measuring the time required for reflected echoes to return from the material surface of interest. The distance is derived from the propagation speed of the energy burst and the elapsed time of the echo travel for the echo returning from the target of interest. Recent instruments of this type use a combination of analog and digital circuits and include a microcontroller. A microcontroller typically consists of a microprocessor, sometimes referred to as a central processing unit, program memory, data memory, and peripheral devices such as analog to digital and digital to analog converters, memory controllers, serial communication ports, timers, etc. As noted above, the supply energy may be very limited. Moreover, in hazardous application environments the sensor supply energy may be very limited, to preclude the possibility of igniting flammable substances.
Signal processing methods that require substantial run time microprocessor activity to accurately perform echo location tasks conflict with the requirement that the device consume minimal amounts of electricity, or result in low measurement update rates. Analog circuit methods for determining echo time of flight can measure efficiently and accurately but can be triggered by transient electrical noise or spurious signals resulting in erroneous measurements.
The present invention is directed to improvements in measurement instruments.